In a large saucepan, melt the butter over medium heat. Whisk in the flour to create a roux, cooking for about 1 minute.
Gradually add the milk and heavy cream, whisking continuously to prevent lumps. Cook until the mixture thickens and starts to bubble.
Lower the heat and add the shredded cheese blend, mustard powder, paprika, salt, and pepper. Stir until the cheese has melted and the sauce is smooth.